Plastic waterproof breathable soles for shoes are already known.
One such sole is disclosed in the commonly assigned U.S. Pat. No. 5,983,524 and EP-0858270, hereby incorporated by reference.
In this case, the sole comprises:
a mid-sole with a membrane made of waterproof breathable material, associated with a lower protective layer made of a material which is resistant to hydrolysis, water-repellent, breathable and/or perforated; a tread made of perforated elastomer, which is perimetrically joined hermetically to the mid-sole.
The waterproof breathable sole disclosed in the commonly assigned U.S. Ser. No. 09/423,299 and EPA-98924244.1, hereby incorporated by reference, is also known; it comprises a preassembled insert, in which there is a waterproof breathable membrane associated with a lower protective layer made of a material which is resistant to hydrolysis, water-repellent, breathable and/or perforated.
The insert is completed by an element which is molded or assembled over it, surrounds the membrane and the protective layer, and is joined hermetically thereto.
The insert is part of a mid-sole and is joined, together with said mid-sole, to a tread made of perforated plastics, which is molded or assembled over them.
The sole disclosed in U.S. Pat. No. 5,598,644 and EP-0619959 by the same Applicant, hereby incorporated by reference, is also known; it comprises a leather tread which is at least partially covered, in an upward region, by means of a mid-sole constituted by a waterproof breathable membrane and by a perimetric plastic element which is sealed to the membrane and is monolithically associated with the tread.
If the tread is made of perforated plastics, the protective element arranged below the membrane is designed to protect said membrane from perforation by foreign objects that have passed accidentally through the holes.
Although the above described soles have been commercially available for years and are unanimously acknowledged to ensure correct exchange of heat and water vapor between the microclimate inside the shoe and the external one, they are not free of drawbacks, including in particular the tendency of the membrane to tear because of the difference between its traction elasticity modulus and that of the tread, with which it is assembled monolithically at its peripheral region.
The membrane is in fact usually made of expanded polytetrafluoroethylene, a material that has very limited elasticity, while the sole, made of rubber or other polymeric material or leather, must be very elastic and flexible owing to its inherent properties and to the requirements of the application.
Accordingly, the membrane is unable to absorb the stresses induced in it by the flexural deformations of the sole in movements during use and accordingly tends to tear and lose its waterproof properties.
The protective element arranged below the membrane, which is usually made of polyester felt, has been found unable, by itself, to absorb the tensions induced by the deformations of the sole.